The present invention relates to a process and a plant for UHT sterilisation of milk-based produces.
It is known that UHT sterilisation, if followed by packaging under aseptic conditions, enables produces to be obtained that can be preserved for long periods of time at room temperature without the produce being damaged.
UHT sterilisation takes place following two main technologies: direct sterilisation by steam injection into the produce; indirect sterilisation with heat exchangers over the whole technological cycle. In the case of steam injection, it is obtained either with an injector directly admitting steam to the milk mass in transit or with an infuser, consisting of a chamber into which the milk falls forming a film while steam is being admitted to the chamber.
The direct-injection technology enables better organoleptic features to be obtained as compared with the indirect technology, and also the decay to which the produce is submitted during the sterilisation process to be reduced. On the other hand, energy and water consumption in a direct sterilisation plant is much higher than that in a corresponding indirect sterilisation plant.
The first examples applying the direct technology go back to the beginning of the sixties. In the progress of years the direct technology has evolved to obtain better performance and less waste, and different embodiments starting from the base process have been developed. In the classic technology of direct steam injection, the produce is fed to the plant at low temperature, i.e. 4-8.degree. C. Then a pre-heating to 70-80.degree. C. is carried out by use of the regeneration heat from the milk-cooling circuit after sterilisation. By an injector or an infuser the produce is then brought to 140-145.degree. C. by direct transfer of latent heat from steam to produce (the produce being consequently diluted in proportion to the acquired heat). Then there is a standing period for sterilisation at the highest temperature for 3-6 seconds and subsequently the produce is introduced into a vacuum chamber where steam added during sterilisation is drawn out, by making it condense on an appropriate water-cooled surface condenser. After homogenisation of the produce at a degassing temperature of 70-80.degree. C., the produce is definitively cooled in a heat exchanger providing water recirculation, which water is intended for pre-heating the produce at its entry.
Such a plant has the unquestionable advantage of being of simple structure and requiring washing cycles relatively spaced out in time (even 30-36 hours of uninterrupted production before washing). In addition, the sterilisation step and subsequent cooling are very quick and on coming out of the degassing device the produce has already reached the homogenisation temperature. A high precipitation of the casein and proteins however occurs both in the high-temperature section of the plant and during the final packaging.
In an attempt to obviate the above drawbacks some alternative versions to the classic technology have been proposed. For example, a pre-heating to 90.degree. C. of the produce fed to the plant in this case too at a temperature of 4-8.degree. C. has been suggested. This heating is advantageously carried out utilising the regeneration heat from the milk-cooling circuit after sterilisation. After a standing period for stabilisation, or stabilisation pause, aiming at reducing protein precipitation, a further indirect pre-heating at 120.degree. C. is carried out. After that, the produce is submitted to heating at 140-145.degree. C. by an injector or an infuser, then to a sterilisation pause at the highest temperature for 3-6 seconds and a first cooling with the heat exchanger at 100.degree. C. After that, the produce follows the classic treatment, involving introduction of same into the vacuum chamber for drawing out the steam added during sterilisation, subsequent homogenisation at the degassing temperature of 70-80.degree. C. and final cooling by exchanger with water recirculation which will carry out pre-heating of the produce being introduced.
While a reduction in energy consumption is achieved, on the other hand a worsening in the organoleptic qualities of the produce occurs, as well as a shorter working time between one washing and the next one (12-15 hours at most), due to the exchangers becoming quickly dirty at high operative temperatures.
A further alternative solution to this second method has been also proposed, in which degassing and homogenisation are provided to take place before sterilisation, in turn carried out with an infuser. However no important advantages exist as compared with the preceding solution. Such a solution is proposed in U.S. Pat. No. 4,310,476 and U.S. Pat. No. 4,419,301.
It is a general object of the present invention to obviate the above mentioned drawbacks by providing a process and a plant for production of milk and the like sterilised at high temperature, which enable a high organoleptic quality to be obtained together with a low cost for servicing of the plants putting this process into practice.